Manifolding system with four-barrel carburetor



P. F. KEYDEL July 26, 1955 MANIFOLDING SYSTEM WITH FOUR-BARRELCARBURETOR 2 Sheets-Sheet 1 Filed Dec.

July 26, 1955 P. F. KEYDEL 2,713,857

MANIFOLDING SYSTEM WITH FOUR-BARREL CARBURETOR Filed Dec. 1, 1951 2Sheets-Sheet 2 V gmenbr 5 fay/42% slimme 5 United States Patent 0MANIFOLDIN G SYSTEM WITH FOUR-BARREL CARBURETOR Paul F. Keydel, Detroit,Mich., assignor to General Motors Corporation, Detroit, Mich., acorporation of Delaware Application December 1, 1951, Serial No. 259,445

13 Claims. (Cl. 123-122) This invention relates to charge formingdevices for engines and has a particular relation to an inlet manifoldfor use with a compound or other carburetor for supplying explosivemixtures to cylinders in V-type or other internal combustion engines.

It is proposed to increase the volumetric efiiciency of engines and tomore nearly equalize the distribution of combustible mixture among thevarious cylinders thereof by employing a compound carburetor forsupplying different combustible mixtures under different conditions ofoperation to the middle portion of a main distribution passage, theopposite ends of which are formed to provide ports for supplying themixtures to a group of cylinders of the engine.

The compound carburetor has a primary mixture supply passage whichsupplies combustible mixture to the main distribution passage throughoutall conditions of operation of the engine and has a secondary mixturesupply passage which supplies combustible mixture to the maindistribution passage under conditions of operation of the engine whenthe mixture supplied by the primary mixture supply passage is inadequatein quantity or improper in fuel and air ratio or both.

In order to improve the distribution of the fuel mixture supplied by theprimary mixture supply passage it is proposed to locate the junctionbetween the primary supply and main distribution passages nearer themiddle of the main distribution passage than the junction between thesecondary supply passage and the main distribution passage. Since theprimary supply passage supplies to the main distribution passage all ofthe combustible mixture employed in starting the engine and in operatingthe engine until the engine is warm enough for normal operation, it isproposed to heat the primary mixture supply passage and the junctionthereof with the main distribution passage. A heated charge thereforemay be employed in starting the engine. To accomplish this purpose, thecharge forming device is provided with exhaust or other suitable heatingmeans for heating a wall of the main distribution passage opposite thejunction of the main distribution passage with the primary mixturesupply passage and the wall of the primary supply passage anterior tothe junction referred to. If a carburetor having primary and secondarymixture supply passages is formed as a single heat conductive unit, thenthere may be some heating of the secondary mixture supply passage andthe wall of the main distribution passage opposite the junction betweenthe two passages. But this heating will not noticeably affect thevolumetric efliciency of the engine under normal operating conditions.The heat supplied will be mostly absorbed by the mixture supplied by theprimary mixture supply passage and little will be conducted to thesecondary mixture supply passage. Also it is proposed to cut off thesupply of heat to the heating means referred to after the engine issufliciently warmed for normal operation. Since the secondary mixturesupply passage does not supply mixture to the main distribution passageduring the time when heat is supplied to the'heating 2,713,857 PatentedJuly 26, 1955 means, there will be little heating of the secondarypassage and the junction thereof with the main distribution passage whenthe secondary passage later is employed in supplying mixture to the maindistribution passage.

In the present instance it is proposed to employ the charge formingdevice with a V-type engine in which the cylinders are divided into twogroups of four cylinders each and in which one main distribution passagesupplies mixture to each group of four cylinders. In such anarrangement, each main distribution passage is supplied with a primarymixture supply passage and a secondary mixture supply passage, thejunctions thereof with the main distribution passages being located aspreviously described.

It is also proposed to employ liquid fuel collecting sumps in the wallsof the main distribution passages, these being located directly oppositethe primary mixture supply passages. These liquid fuel sumps are heatedby the heat supplied to the junctions between the passages to preventunvaporized liquid from being delivered to the cylinders of the engine.

Particularly with V-type eight cylinder engines it is proposed to employa cross-over passage between the exhaust passages of one or more of themiddle cylinders of each group of cylinders of the engine. Thiscross-over passage may be formed integrally with the inlet manifold unitor body in which the main distribution passages and the primary andsecondary mixture supply passages are formed. The walls of the heatingor cross-over passage and the main distribution passages are providedwith fins for extending the surface thereof and these fins areconstructed and arranged in such a way that uniform heating of the maindistribution passages will be provided.

In the drawings:

Figure 2 is a fragmentary end elevational view of an engine having acharge forming device embracing the principles of the invention.

Figure 1 is a plan View with parts thereof broken away to show theinterior of the structure of the inlet manifold part of the chargeforming device illustrated by Figure 2.

Figure 3 is a fragmentary plan view of the inlet manifold disclosed byFigure 1, with parts thereof broken away and shown in section at thejunctions between the primary supply and main distribution passages ofthe manifold.

Figure 4 is a fragmentary vertical sectional view of the inlet manifoldand the heating passage therefor taken substantially in the plane ofline 44 of Figure 1.

Figure 5 is another vertical sectional view of the manifold illustratedby Figure 1. Figure 5 is taken along the broken dot and dash line onFigure 1 looking in the direction of the arrows thereon.

Figure 6 is a side elevational view of the manifold disclosed byFigure 1. Figure 6 shows certain parts of the manifold broken awayacross the heating, main distribution and supply passages of themanifold in order better to illustrate the interior construction of themanifold structure.

The engine 10 employed in practicing the invention is a V-type eightcylinder internal combustion engine especially applicable for theoperation of motor vehicles. The engine 10 includes a block 11 havingbanks of cylinders 12 and 13 provided with heads 14 and 16 respectiveiy.The cylinders in the banks 12 and 13 are provided with exhaust manifolds17 and 18 adapted to discharge the exhaust gas from the engine to theatmosphere through the exhaust pipes 19 and 21 respectively. The exhaustpipe 19 is adapted to be cut off by a thermostatically controlled heatcontrol valve indicated at 20 which will open when the engine is warm topermit the exhaust gas to escape through the exhaust pipe 19.

The engine it) is provided With an inlet manifold 22 having outlet ports23 adapted to be connected to inlet passages formed in the heads 14 and16 and leading to the different cylinders of the engine 10. The manifold22 is supplied with combustible mixtures for operating the engine by acompound carburetor indicated at 26. The manifold 22 is preferablyformed of an integralcast body having formed therein a pair of maindistribution passages 27 and 28. The main distribution passages 27 and28 are approximately of equal length and cross-sectional area, extend inparallel relation to one another and the banks of cylinders of theengine 10, and the distribution passage 27 is arranged somewhat abovethe distribution passage 28 so that each passage may be readily employedfor supplying mixtures to certain cylinders in each of the banks ofcylinders of the engine. The distribution passage 27 has branch passages29 and 31 extending in opposite directions from one end thereof forsupplying an end cylinder of the bank 12 and a middle cylinder of thebank 13 respectively. The opposite end of the passage 27 is alsobranched at 32 and 33 for supplying the opposite end cylinder of thebank 12 and the remaining middle cylinder of the bank 13 respectively.The main distribution passage 28 is also provided with oppositelydisposed branches 34 and 36 for supplying respectively a middle cylinderof the bank 12 and an end cylinder of the bank 13. The opposite end ofthe main distribution passage 28 is similarly branched at 37 and 38 forsupplying respectively the remaining middle cylinder of the bank 11 andthe remaining end cylinder of the bank 13. The branches 29, 31, 32, 33,34, 36, 37 and 38 of the main distribution passages 27 and 28 are all ofsubstantially equal length and cross-sectional area so as to providesubstantially equal distribution of combustible mixture to all of thecylinders of the engine 10. It will be apparent that the maindistribution passage 27 supplies two end cylinders of the bank 12 andtwo middle cylinders of the bank 13, while the main distribution passage28 supplies two middle cylinders of the bank 12 and two end cylinders ofthe bank 13. The cylinders of the engine therefore are divided into twodifferent groups, one group being supplied by the main distributionpassage 27, and the other by the main distribution passage 28. Since themain distribution passage 27 is located somewhat above the maindistribution passage 28, it will be apparent that the branch passagesextending from each of the main disribution passages can cross over withrespect to one another Without unduly distorting any of the passages.Also, it will be apparent that the main distribution passages 27 and 28are curved to a slight extent toward one another and at the oppositeends thereof so as to permit the main distribution passages to join thebranch passages at approximately the middle of the combined lengths ofthe branch passages.

The middle parts of each of the main distribution passages 27 and 28 areadapted to be supplied with combustible mixtures by one of a pair ofprimary mixture supply passages 39 and one of a pair of secondarymixture supply passages 41. The primary and secondary mixture supplypassages 39 and 41 terminate in the manifold 22 and originate in thecompound carburetor 26 where the carburetor jets form separate mixturesfor each of the pairs of passages 39 and 41. The primary supply passages39 are controlled by an automatic choke valve mechanism indicated at 42which is adapted to restrict the passages 39 anterior to the carburetorjets for the purpose of supplying a richer mixture during the startingof the engine than at other times during engine opertaion. Thecarburetor 26 also is provided with the usual butterfly-type throttlevalves for each of the passages 39 and 41. The passages 39 may beprovided with throttle valves secured to the same shaft and operating asa unit for controlling the supply of mixture through the primary mixturesupply passages 39. The secondary mixture supply passages 41 also may beprovided with butterfly-type throttle valves on the same shaft and whichlikewise operate as a unit within the carburetor 26 for controlling thesupply of secondary mixture to the main distribution passages 27 and 28.The carburetor 26 is provided with a throttle operating mechanism 43 bywhich the throttle unit for controlling the passages 39 is correlatedwith the throttle unit for controlling the passages 41. The throttleunit for the passages 41 is held in closed position until the throttleunit for the passages 39 is opened to the desired extent and until thechoke valve for the carburetor is fully opened. The throttle controldevice 43 operates the carburetor 26 in such a way that a combustiblemixture will be supplied to the main distribution passages 27 and 28through the primary mixture supply passages 39 during all conditions ofoperation of the engine 10 and through the secondary mixture supplypassages 41 only during certain conditions of operation of the engine10. Since the primary mixture supply passages 39 supply mixtures to themain distribution passages 27 and 28 under all operating conditions ofthe engine 10, it is considered preferable to locate the junctionbetween the passages 39 and the passages 27 and 28 somewhat nearer themiddle of the passages 27 and 28 than the junctions between the passages41 and the passages 27 and 28. This will result in a somewhat betterdistribution of combustible mixture to all of the cylinders of theengine 10 during low speed operation of the engine 10 and during thetime when mixture is being supplied to the main distribution passages 27and 28 only through the primary supply passages 39.

Since the primary supply passages 39 are exclusively employed insupplying combustible mixtures to the main distribution passages 27 and28 for starting the engine 10, it is considered desirable to heat themixtures entering the engine through the passages 39. To accomplish thisthe manifold '22 is formed in such a way as to provide a cross-overheating passage indicated at 44. The passage 44 has ports 46 and 47formed at the opposite ends thereof which are adapted to communicatewith exhaust passages formed in the heads 14 and 16 and leading from thetwo middle cylinders of the banks 12 and 13. When the engine 10 is coldand the heat control valve 21 is closed, it will be apparent that all ofthe exhaust from the cylinders in the bank 12, which normally would bedischarged from the manifold 17 through the exhaust pipe 19, will becompelled to flow in a reverse direction. The exhaust from the endcylinders of the bank 12 will flow into the exhaust manifold 17, then ina reverse direction through the exhaust passages for the two centralcylinders of the bank 12 and from which all of the exhaust gas from allof the cylinders of the bank 12 will be discharged through the exhaustpassage leading to the port 46. From the port 46 the exhaust gas fromall of the cylinders of the bank 12 will flow through the heatingpassage 44, from the port 47 and into a passage in the bank 13 leadingto the two middle cylinders of the bank 13. From the exhaust passage forthe two middle cylinders of the bank 13 the exhaust gas from the bank 12will be discharged into the exhaust manifold 18 with the exhaust gasfrom the cylinders of the bank 13. From the exhaust manifold 18 all ofthe exhaust gas from the cylinders of the banks 12 and 13 will bedischarged from the engine through the ex haust pipe 21. When the engineis warm enough for normal operation and the heat control valve 21 opens,then the exhaust gas from the cylinders of the banks 12 and 13 willfollow the paths of least resistance, which will be respectively throughthe exhaust manifolds 17 and the exhaust pipe 19 and the exhaustmanifold 18 and the exhaust pipe 21. It will be apparent that under suchconditions the fiow of exhaust gas through the passage 44 will bereduced to a minimum.

The heating passage 44 is formed in the manifold 22 in such a way as toextend across the lower parts of the main distribution passages 27 and28 and to expose the walls of the main distribution passages 27 and 28to the heat in the exhaust gas that will flow through the passage 44when the engine is cold. The heating passage 44 has the walls thereofarranged in such manner that the heating passage will extend across thelower parts of the main distribution passages 27 and 28 just below thejuctions between the main distribution passages 27 and 28 and theprimary mixture supply passages 39. The heating passage 44 spreads outfrom each of the ports 46 and 47 in such a way as to provide a broadermore shallow passage in the region where the passage 44 extends beneaththe main distribution passages 27 and 28. The broader part of thepassage 44 is indicated by the numeral 48. Directly above the middle ofthe broader part 48 of the passage 44 and immediately opposite thejunctions between the primary passages 39 and the main distributionpassages 27 and 28, the lower walls 49 and 51 of the main distributionpassages 27 and 28 are formed in such a Way as to provide depressions orliquid fuel collecting sumps indicated at 52 and 53 respectively. Theupper surfaces of the lower walls 49 and 51 are extended for heattransfer purposes by the provision of fins 54 and 56. These fins arespaced from one another across the width of the sumps 52 and 53respectively, project slightly above the upper extremities of the sumps52 and 53 and extend in parallel relation to the passages 27 and 28respectively.

It will be apparent that the heavier ends of the fuel in the mixturesupplied by the passages 39 to the main distribution passages 27 and 28will be collected in the sumps 52 and 53, particularly when the engineis cold. Thereafter the liquid collected in the sumps 52 and 53 will bevaporized by the heat transferred from the passage 44 through the walls49 and 51 and through the extended surfaces thereof provided by the fins54 and 56. It will be apparent from Figure 6 that the downwardprojections of the walls 49and 51 of the passages 27 and 28respectively, which provide the sumps 52 and 53 respectively, terminateat the ends thereof between the oppositely disposed walls forming theside walls of the Wider part 43 of the heating passage 44.

From Figures 1 and 5 it will be apparent that from the port 46 thepassage 44 not only increases in Width, but extends upwardly in such aWay as to discharge the exhaust gas from the bank of cylinders 12 into apocket 62 and directly against the adjacent side wall 57 of the maindistribution passage 27. The width of the passage 44- at the pocket 62and adjacent the side wall 57 is great enough to heat the side wall 57somewhat beyond the ends of the sump 52. It will be apparent that themain distribution passage 27 adjacent the junction thereof with theprimary passage 39 has one side wall and the bottom wall thereofdirectly exposed to the exhaust gas supplied by the passage 44throughout a distance exceeding the length of the sump 52. The widerpart 48 of the passage 44 will direct exhaust gas directly against theside wall 58 of the main distribution passage 28 throughout the entirewidth of the passage section 48. Beyond the main distribution passage 28the heating passage 44 extends upwardly along the opposite side wall 59of the main distribution passage 28 in such a way as to provide anupwardly extending pocket indicated at 61. The pocket 61 is narrowerthan the corresponding pocket 62, which extends along the side wall 57of the main distribution passage 27, but notwithstanding this it will beapparent that the exhaust gas within the passage 44 will be applied toone side wall 57 and the bottom wall 49 of the main distribution passage27, whereas exhaust gas will be applied to two side walls 58 and 59 andthe bottom wall 51 of the main distribution passage 28. The side wall 57and the bottom wall 49 are therefore provided with outwardly projectingfins indicated at 63 to compensate for the greater surface of the wallsof the main distribution passage 28 which is exposed to the exhaust gaswithin the passage 44.

In order to provide means for directly heating the primary supplypassages 39 during the time when the engine is being started, themanifold 22 is formed in such a way as to provide passages 64 and 66which extend to the upper surface of the manifold 22 from the pockets 62and 61. respectively. In order to join the upper ends of the passages 64and 66 to provide a bypass passage 67 for heating the primary mixturesupply passages 39, the lower wall 68 of the carburetor 26, which isadapted to engage the upper wall 69 of the manifold 22, is channeled insuch a way as to provide a sinuous passage indicated at 71. It will benoted from Figure 1 that the passage 71 extends around the side of theprimary mixture supply passages 39, which is opposite the secondarymixture supply passages 41 and in the vicinity of and just below theidling passage 72 of the carburetor 22. It will be apparent that theexhaust gas which is trapped in the pocket 62 will flow through theby-pass passage 67 and will heat the mixture in the primary mixturesupply passages and in the idling passage 72 before the mixture reachesthe main distribution passages 27 and 28. It will also be apparent thatthe exhaust gas flowing through the passage 44 will heat the walls 57,49, 58, 51 and 59 of the main distribution passages 27 and 28 in thevicinity of the liquid fuel collecting sumps 52 and 53 and at thejunctions between the main distribution passages 27 and 28 and theprimary mixture supply passages 39.

No means is provided for directly heating the secondary mixture supplypassages 41, nor the junctions between the passages and the maindistribution passages 27 and 28. Although some heat may be transferredthrough the walls of the manifold between the passages, it will beapparent that the mixture supplied by the secondary mixture supplypassages 41 will not be heated to any great extent, either within thepassages 41 or at the junctions between the passages 41 and the maindistribution passages 27 and 28. The reason for this is that underordinary operating conditions in the engine 10 no mixture will besupplied by the secondary mixture supply passages 41 until after theheat control valve 21 has been opened. Consequently when mixture issupplied by the secondary mixture supply passages 41, there will belittle exhaust gas actually supplied to the manifold 22 through theheating passage 44.

I claim:

1. An inlet manifold for engines comprising a body having a pair of maindistribution passages formed therein and having branch passages at theopposite ends of said main distribution passages extending in oppositedirections therefrom, primary and secondary mixtures supply passagesformed in said body and connected to said main distribution passagesintermediate the ends of said main distribution passages, and wallsforming an exhaust gas heating passage extending transversely acrosssaid main distribution passages on the sides of said main distributionpassages opposite said mixture supply passages and in position to heatthe walls of said main distribution passages directly opposite saidprimary mixture supply passages to a greater extent than the Walls ofsaid main distribution passages opposite said secondary mixture supplypassages.

2. An inlet manifold for engines comprising a body having a pair of maindistribution passages formed therein and branch passages for said maindistribution passages extending opposite directions from the oppositeends of said main distribution passages, said main distribution passagesbeng arranged in side by side relation with one passage above the other,a heating passage formed in said body and extending transversely acrossthe lower part of said main distribution passages intermediate the endsthereof, said treating passage being formed around the lower walls ofsaid main distribution passages to expose two side walls and the bottomwall of the lower of said main distribution passages and the bottom walland a side wall of the other of said main distribution passages, and finmeans formed on the side wall and bottom wall of said upper maindistribution passage for equalizing the heat transfer rate through saidwalls of said main distribution passages exposed to heating fluid insaid heating passage.

3. An inlet manifold for engines comprising a body having maindistribution passages formed therein in parallel relation to oneanother, said body being formed to provide branch passages extending inopposite directions from opposite ends of said main distributionpassages for supplying combustible mixture from said main distributionpassages to different groups of cylinders of an engine, a primarymixture supply passage and a secondary mixture supply passage formed insaid body and communicating with each of said main distribution passagesintermediate the ends thereof, a liquid fuel collecting sump formed in awall of each of said main distribution passages in opposed relation tosaid primary mixture supply passages, and a heating passage formed insaid body and extending transversely across said main distributionpassages and exposing the walls of said main distribution passages andsaid liquid fuel sumps to the heating fluid supplied by said heatingpassage.

4. An inlet manifold for engines comprising a body having a pair of maindistribution passages formed therein for supplying combustible mixturesto different groups of cylinders of an engine, said main distributionpassages having branch passages formed in said body and extending inopposite directions from the ends of said main distribution passages,primary and secondary mixture sup ply passages formed in said body andcommunicating with each of said main distribution passages, liquid fuelsumps formed in the walls of said main distribution passages in oppositerelation to said primary mixture supply passages, fins formed in thebottom walls of said main distribution passages in spaced relation toone another and T extending in parallel relation to said maindistribution passages and dividing said sumps into a plurality ofparallel compartments, and a heating passage formed in said body andextending across said walls of said main distribution passages oppositesaid fins said sumps formed in said walls and limited in extentsubstantially to the parts of said walls containing said fins and saidsumps.

5. An inlet manifold for engines comprising a pair of main distributionpassages for supplying combustible combustible mixtures to differentgroups of cylinders of an engine, a primary and a secondary mixturesupply passage communicating with each of said main distributionpassages intermediate the ends thereof, means controlling the supply ofcombustible mixtures by said primary mixture supply passages under allconditions of operation of said engine, means preventing the supply ofcombustible mixtures through said secondary mixture supply passages tosaid main distribution passages under certain conditions of operation ofsaid engine, a heating passage formed in. said manifold and exposing tothe heating fluid therein the walls of said main distribution passagesadjacent said primary and secondary supply passages, said walls of saidmain distribution passages opposite said primary supply passages beingexposed to said heating passage to a greater extent than the walls ofsaid main distribution passages opposite said secondary supply passages,and tin means formed upon the walls of said main distribution passagesin opposed relation to said heating passage.

6. A charge forming device for engines comprising a pair of maindistribution passages, primary mixtures supply passages connected tosaid main distribution passages for supplying combustible mixtures tosaid main distribution passages under certain conditions of operation ofsaid engine, secondary mixture supply passages connected to said maindistribution passages for supplying combustible mixtures to said maindistribution passages under other conditions of operations of saidengine, a heating fiuid passage formed in said device and extendingtransversely across said main distribution passages in opposed relationto said primary mixture supply passages, and a by-pass passage formed insaid device on the side of said primary mixture supply passages oppositesaid secondary mixture supply passage and having the opposite endsthereof connected to said heating passage on opposite sides of said maindistribution passages, said secondary mixture supply passages havingwall surfaces in directly opposed relation to said by-pass passage anddirectly exposed to the atmosphere surrounding said device.

7. A charge forming device for engines comprising a body having a pairof main distribution passages formed therein, primary and secondarymixture supply passages for supplying combustible mixtures to said maindistribution passages and communicating with each of said maindistribution passages intermediate the ends thereof, a heating passageformed in said body and extending transversely across said primarymixture supply passages on the side of said primary mixture supplypassages opposite said secondary mixture supply passages, said secondarymixture supply passages having wall surfaces in directly opposedrelation to said heating passage and directly exposed to the atmospheresurrounding said device, and means for supplying heating fiuid to saidheating passage.

8. A charge forming device for engines comprising a body having maindistribution passages formed therein for supplying combustible mixturesto different groups of cylinders of an engine, primary and secondarymixture supply passages formed in said body and communi- 3 eating witheach of said main distribution passages intermediate the ends of saidmain distribution passages, and means for heating sai'd maindistribution passages opposite said primary mixture supply passages to agreater extent than said main distribution passages opposite saidsecondary mixture supply passages.

9. A charge forming device for engines comprising a body having a pairof main distribution passages formed therein for supplying combustiblemixtures to different groups of cylinders of an engine, primary andsecondary mixture supply passages formed in said body and connected tosaid main distribution passages intermediate the ends of said maindistribution passages, and means for heating said primary mixture supplypassages to a greater extent than said secondary mixture supplypassages.

10. A charge forming device for engines comprising a body having aplurality of parallel main distribution passages for supplyingcombustible mixtures to different groups of cylinders of an engine, aprimary mixture supply passage communicating with each of said maindistribution passages for supplying combustible mixtures to said maindistribution passages under certain conditions of operation of saidengine, a secondary mixture supply passage for eachof said maindistribution passages for supplying combustible mixtures to said maindistribution passages under dilferent conditions of operation of saidengine, and a heating passage formed in said body adjacent said primarymixture supply passages and remote from said secondary mixture supplypassages.

11. An inlet manifold for engines comprising a body having apain ofparallel main distribution passages formed therein, a primary and asecondary mixture supply passage formed in. said body and communicatingwith each of said main distribution passages intermediate the ends ofsaid main distribution passages, said mixture supply passages beingformed in said body with the junctions between said primary mixturesupply passages and said main distribution passages being nearer themiddle of said main distribution passages than the junctions betweensaid secondary mixture supply. passages and said main distributionpassages.

12. A charge forming device for engines comprising a body having a pairof main distribution passages formed therein for supplying combustiblemixtures to different groups of cylinders. of an engine, a primary and asecondary mixture supply passage communicating with each of said maindistribution passages intermediate the ends of said main distributionpassages, said primary mixture supply passages having junctions withsaid main distribution passages nearer the middle of said maindistributionthan the junctions between said secondary mixture supplypassages and said main distribution passages, and means for heating saidjunctions between said primary mixture supply passages and said maindistribution passages to a greater extent than said junctions betweensaid secondary mixture supply passages and said main distributionpassages.

13. A charge forming device for engines comprising a body having a pairof main distribution passages formed therein for supplying combustiblemixtures to diflerent groups of cylinders of an engine, primary andsecondary mixture supply passages communicating with each of said maindistribution passages intermediate the ends of said main distributionpassages, the junctions between said pri- 10 mary mixture supplypassages and said main distribution passages being nearer the middle ofsaid main distribution passages than the junctions between the saidsecondary mixture supply passages and said main distribution passages,and means for differently heating said primary and secondary mixturesupply passages.

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